Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session J02: Dynamics of Polymers Under ConfinementFocus Live
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Sponsoring Units: DPOLY DSOFT DCP Chair: Robert Riggleman, University of Pennsylvania |
Tuesday, March 16, 2021 3:00PM - 3:12PM Live |
J02.00001: Comparing Attachment Methods of Surface Bound Chains and Their Impact on Local Glass Transition Dynamics Connie Roth, Michael F Thees, Xinru Huang How polymer chains become bound to interfaces are fundamental to surface modification, adhesion, and reinforcement mechanisms. Efforts to investigate adsorbed chains in melt films and polymer nanocomposites frequently rely on solvent washing to expose such near-surface, "bound layer" chains. By comparing and contrasting surface bound chains in melts and solutions, we leverage the wealth of information on polymer adsorption in solution developed over several decades to inform us about polymer conformations in the melt. We find that the adsorbed layer thickness remaining from the solvent rinsing of melt films is entirely determined by the solvent washing conditions used, with only the time in solution needed to reach this final adsorbed amount affected by prior treatment of the melt film. These observations are consistent with surface diffusion and exchange measurements of adsorbed chains in solution demonstrating that surface bound chains are highly mobile, even for the case of strong adsorption. We compare how these different populations of surface bound chains alter the local glass transition temperature of neighboring polymer chains in bulk films, and compare them to end-tethered grafted chains. |
Tuesday, March 16, 2021 3:12PM - 3:24PM Live |
J02.00002: Dynamical gradients, barrier factorization and interface coupling in
thick and thin films of glass-forming liquids Kenneth Schweizer, Anh D. Phan A force-level microscopic theory is constructed for the heterogeneous dynamics of glassy polymer liquids in free standing films. The activated event involves cage scale hopping coupled with a longer range collective elastic distortion of the medium. For thick films, weaker caging constraints at the vapor surface are dynamically transferred well into the bulk, and the elastic displacement field at the surface is cut off. A double (single) exponential form of the alpha time (Tg) gradient, near factorization of the temperature and spatial location dependences of the activation barrier, and position-dependent power law decoupling of the relaxation time from its bulk analog are predicted. The ideas are generalized to thin films resulting in an interference of weakened caging constraints from the two surfaces plus a finite size confinement modification of elasticity effects. Temperature-dependent consequences with decreasing film thickness include additional acceleration and flattening at the film center of the alpha time gradient, and large reduction of the film averaged effective barrier. The results appear to be consistent with simulations and experiment. The reliability of a linear superposition approximation of exponential gradients from a thick film have been analyzed. |
Tuesday, March 16, 2021 3:24PM - 3:36PM Live |
J02.00003: Nanoconfined Benzyl Methacrylate Polymerization: Kinetics, Thermodynamics, and Chain Entropy Loss Chunhao Zhai, Bryan D Vogt, Sindee L Simon Nanoconfinement is known to affect the behavior of polymeric materials, including changes in the Tg and relaxational dynamics. However, the effect of nanoconfinement on the kinetics and thermodynamics of polymerization is less well investigated and understood. Here, the nanoconfined free radical polymerization of benzyl methacrylate is investigated using differential scanning calorimetry. Controlled pore glass (CPG) and ordered mesoporous carbon with pore sizes ranging from 3 to 8 nm are used as the confinement media. The propagation rates in CPG increase as pore size decreases and is proportional to the surface silanol concentration, whereas the rates in the carbon mesopores decrease with reciprocal pore diameter. Autoacceleration is more pronounced under nanoconfinement, and the nanoconfined polymer has higher molecular weight compared to that synthesized in bulk – both of these effects are attributed to a decrease in the nanoconfined termination rate arising from a decrease in diffusivity of the confined chains. We also explore the thermodynamics of the nanoconfined polymerization and find that the entropy loss due to confinement of chains scales with molecular weight to the first power, consistent with theoretical predictions for the weak confinement regime. |
Tuesday, March 16, 2021 3:36PM - 3:48PM Live |
J02.00004: Comparing Local Glass Transition by Fluorescence with Physical Aging by Ellipsometry Within Rubbery-Glassy PnBMA/PS Bilayer Films James H Merrill, Jennifer A McGuire, Michael F Thees, Connie Roth Expanding our ellipsometry analysis, we were recently able to isolate the impact of a single glassy-rubbery interface on the physical aging response of glassy polystyrene (PS) within bilayer films of poly(n-butyl methacrylate) (PnBMA) atop PS, avoiding competing free surface effects. Surprisingly, no change in the glassy PS aging response was observed down to layer thicknesses of 150 nm PnBMA / 75 nm PS, the thinnest we were able to measure. At first glance, these results seem at odds with our group’s previous findings of large local glass transition temperature Tg(z) changes near glassy-rubbery polymer interfaces in semi-infinite systems. Here we investigate this directly by performing fluorescence measurements to determine the PS Tg response within PnBMA/PS bilayer films with equivalent geometry to those used for the physical aging studies. We have confirmed that the PnBMA/PS interface imparts a Tg reduction to the underlying PS layer that is larger than that caused by the free surface in single layer PS films. For example, 75 nm PS on silica capped by 150 nm PnBMA shows an ≈5 K decrease in Tg relative to bulk. We compare fluorescence measurements of the local Tg(z) profile within 150 nm PnBMA / 75 nm PS bilayer films with the physical aging results obtained by ellipsometry. |
Tuesday, March 16, 2021 3:48PM - 4:00PM Live |
J02.00005: Distribution of Relaxation Dynamics in Polymer Nanocomposites: Wetting and Chain Packing Effects Emmanuel Mapesa, Dayton P Street, S. Michael Kilbey,II, Joshua Sangoro Broadband dielectric spectroscopy and differential scanning calorimetry are used to study dynamics and glass transition temperature of poly(methyl methacrylate)/silica nanoparticle (NP) composites. By systematically examining nanocomposites based on non-functionalized SiO2 NPs dispersed in PMMA matrices and on PMMA-grafted SiO2 NPs in PMMA matrices, we probe the effects of interfacial interactions and confinement in each of these cases on segmental dynamics and Tg. In addition to slower mobility assigned to interfacial relaxations, faster modes – dependent in NP loading level and the molecular weight of the matrix – also arise due to confinement effects. These faster modes are more pronounced in nanocomposites with polymer-grafted nanoparticles. We explain these observations in a unifying way by invoking the concepts of chain wetting and packing in interfacial zones around nanofillers. As a result, these insights provide a possible molecular understanding of well-known macroscopic phenomena in polymer nanocomposites, which may inform efforts to strengthen materials or tailor the nanocomposites for specific applications. |
Tuesday, March 16, 2021 4:00PM - 4:12PM Live |
J02.00006: To Understand Film Dynamics Look to the Bulk Jane E Lipson, Ronald White In recently published work1 we show that shifts in dynamics of confined systems relative to that of the bulk material originate in the properties of bulk, and exhibit the same form of behavior as when different bulk isobars are compared. By applying our Cooperative Free Volume (CFV) model, employing the Locally Correlated Lattice (LCL) equation of state, we connect in a formal and quantitative way the impact of sample nanoconfinement on segmental dynamics to the effect of changing the thermodynamic state of a bulk sample. This analysis succeeds because control of temperature and control of density in altering dynamic response are multiplicatively linked. The result is that our model leads to predictive power: characterization of bulk behavior allows us to anticipate how changing pressure and/or film thickness will affect sample free volume, local activation energies, and cooperativity requirements. This talk will provide a clear presentation of the above, and show how the work leads to a deeper understanding of what drives segmental relaxation under a wide variety of sample conditions. 1. White, R.P.; Lipson, J.E.G. Physical Review Letters (2020) 125, 058002. |
Tuesday, March 16, 2021 4:12PM - 4:24PM Live |
J02.00007: Unconventional thin film pattern relaxation via direct solvent immersion annealing in a solvent mixture Aman Agrawal, Tyler Choate, Kshitij Sharma, Wenjie Wu, Alamgir Karim Direct immersion annealing (DIA) of polymer thin films in a solvent mixture has been found highly effective in block-copolymer ordering, reducing annealing time scales by an order of magnitude. The amount of each solvent in the 'solvent-mixture' is carefully chosen to increase polymer dynamics (with a good solvent) while preventing dewetting (with a non-solvent). We explore the time-scale of selective-solvent diffusion into topographically nanoimprinted films via DIA, by studying the film swelling and the onset of polymer relaxation. Pattern relaxation ("slumping") of PMMA thin films of varying molecular weights (15k - 600k) via DIA is strikingly different from thermal annealing based pattern-relaxation. We observed an "incubation time" corresponding to the diffusion of selective-solvent into the films in presence of a non-solvent. This incubation time changed with the solvent ratio in a non-linear fashion. Surprisingly, the pattern decay dynamics remained unaffected as compared to thermal annealing where the dynamics have been found to couple very well with the slumping temperature. The dependence of solvent diffusion time-scales, film surface tension, as well as the pattern decay dynamics will also be discussed. |
Tuesday, March 16, 2021 4:24PM - 4:36PM Live |
J02.00008: Tuning Physicochemical Properties of Polymers through Extreme Nanoconfinement Ahmad Arabi Shams Abadi, Haonan Wang, Yueli Chen, Ayda Rafie, Mykola Seredych, Vibha Kalra, Zahra Fakhraai Extreme nanoconfinement of a polymer can be achieved by infiltrating a polymer into a highly-loaded nanoparticle (NP) film via Capillary Rise Infiltration (CaRI). Here, we explore the properties of polymers in CaRI films with various strengths of interfacial interactions. Polyethersulfone (PES), poly (2-vinylpyridine) (P2VP), or polystyrene (PS) are infiltrated into SiO2 NPs. Physicochemical properties of these polymers are tuned under various confinement conditions using NPs with various sizes (3−30 nm average pore sizes) and various degrees of NP sintering prior to infiltration. Extreme enhancement of the glass transition temperature (Tg) is observed with Tg increases as high as 106K above bulk Tg for the 11nm sintered NP packings infiltrated by PES. In this case both entropic and enthalpic effects contribute to Tg enhancement. Nanoconfinement leads to substantial improvement in thermal stability of polymers. Upon heating at elevated temperatures, polymers with less char residues are more resistant to degradation. |
Tuesday, March 16, 2021 4:36PM - 4:48PM Live |
J02.00009: Gradient overlap effects in ultra-thin films Asieh Ghanekarade, Daniel Diaz Vela, David Simmons The dynamics of polymers and other glass-forming liquids can exhibit largegradients in the nanoscale vicinity of interfaces. Progress has been made over the last 10 years in understanding the phenomenology of these gradients. An outstanding question is how these gradients behave in extremely thin films, where gradients emanating from distinct interfaces can interact. Here we report on the results of ultra-thin film simulations probing dynamics locally and globally in this gradient-overlap regime. Results point to three general regimes of thin film behavior: one when the film thickness is greater than twice the gradient range; one in which the gradients overlap but do not individually span the film; and an ultra-thin-film limit in which each gradient span fully to the other interface. We report on distinct behaviors in these regimes in terms of the form of the gradients, the presence or absence of a bulk-like domain, and the breadth of the overall film glass transition. These findings have implications for the interpretation of dynamical data in ultra-thin films and for the underlying origin of alterations in dynamics in the nanoscale vicinity of interfaces. |
Tuesday, March 16, 2021 4:48PM - 5:00PM Live |
J02.00010: Dynamics of Capillary Rise Infiltration(CaRI) of entangled polymers into nanoparticle packings R Bharath Venkatesh, Daeyeon Lee Annealing a bilayer of a glassy polymer film and a nanoparticle packing above the glass transition temperature (Tg) of the polymer induces infiltration of the polymer into the interstices of the nanoparticle packing by capillary action. Ellipsometric monitoring of the rising polymer front shows that the infiltration dynamics can be described using the Lucas-Washburn equation. In this work, we probe the effect of extreme nanoconfinement on the infiltration dynamics of entangled polystyrene (molecular weight = 80k – 4M g/mol) into random packings of silica nanoparticles (diameter = 7 and 25 nm). The extent of confinement as given by the ratio of the radius of gyration of polymer to the characteristic pore size in the nanoparticle packing is varied between 2.5 and 50. We show that effective viscosity inferred from the Lucas-Washburn equation is lower than the bulk viscosity in nanoparticle packings with an average pore size of 4 nm; however, we also observe molecular-weight independent viscosity for high molecular weight polystyrene in silica packings with smaller void size (Rpore = 1 nm). We also observe that increasing confinement leads to a significant decrease in the segmental mobility as evidenced by an increase in Tg of polystyrene over 60 K higher than the bulk value. |
Tuesday, March 16, 2021 5:00PM - 5:36PM Live |
J02.00011: Mobility Gradient of Polymer Chains in an Interfacial Region with a Solid Invited Speaker: Keiji Tanaka Polymer composites have been widely used in a variety of engineering fields. The performance and functionality of the composites are closely related to the quality of the interface between polymer and filler. Thus, it is important to study polymer behavior at solid interfaces. We first present how synthetic polymer chains adsorb onto a solid surface. We closely followed the trajectory of a single polymer chain on the surface as a function of temperature using atomic force microscopy. Combining the results with a full-atomistic molecular dynamics simulation revealed that the chain became more rigid on the way to reaching a pseudo-equilibrium state, accompanied by a change in its local conformation from mainly loops to trains. Then, we turn to the relaxation behavior of polymer chains in direct contact with a substrate using interface-sensitive sum-frequency generation (SFG) spectroscopy. The characteristic temperature, at which interfacial chains started to relax, was much higher than the bulk glass transition temperature. The interfacial relaxation dynamics of chains was directly probed as a function of distance from the solid surface using time-resolved evanescent wave-induced fluorescence anisotropy, dielectric relaxation spectroscopy and SFG spectroscopy. We found the presence of the dynamics gradient of chains in the interfacial region with the solid surface. The segmental relaxation of chains in the strongly adsorbed layer at the interface could be slower than that of bulk chains by more than 10 orders. |
Tuesday, March 16, 2021 5:36PM - 5:48PM Live |
J02.00012: Architecture Effect on Conformational Relaxation of Polymer Chains at an Interface Hung K. Nguyen, Daisuke Kawaguchi, Keiji Tanaka Conformational relaxation of polymer chains trapped in a metastable state in close proximity to a solid interface can strongly affect the physical properties of polymer nanocomposites. We here show an effect of thermal annealing on the chain conformation of polystyrene (PS) with different architecture near at the solid interface using sum-frequency generation spectroscopy. Relaxation dynamics of PS chains strongly adsorbed on the substrate surface is discussed. PS chains exhibited much slower relaxation dynamics with a star-shaped architecture than with a linear one. This finding is in accordance with a scenario that the entropic interaction with the solid surface is stronger for a star-shaped macromolecule than for a linear one. |
Tuesday, March 16, 2021 5:48PM - 6:00PM Live |
J02.00013: Enhanced Free Surface Mobility Facilitates Release of Free-volume Holes in Thin-film Polymer Glasses Hao Zha, Xinping Wang, Biao Zuo The existence of holes in glass-former liquid and the concept of free volumes have been recognized as the underpinning of molecular dynamics of polymer glasses. Annihilation of the holes, through various mechanisms, is relevant to the volume contraction and vitrification of the supercooled liquid. In the nanoconfined geometries, the process of free volume loss through diffusion to the surface is very much appealing, as the finite size scale eases hole removal by reducing the diffusional distance. Herein, we explored the free volume diffusion dynamics by investigating the glass transition temperature (Tg) changes in thin poly (ethylene terephthalate) (PET) films due to variations in the fraction of free area at the film surfaces — associated with enhanced molecular mobility, and by modeling using the surface-mediated diffusion model. Our experimental and modeling results showed that the enhanced mobility in the free area on the surface promotes the hole diffusion in thin films, resulting in a decrease of Tg of PET film with an increase of the fractions of these areas. Also, the activation energy for free volume diffusion decreased with the reduction of film thickness, demonstrating that free volume diffusion in thin films can be modulated by free surface mobility. |
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